Why isn't my voice search available when the Bluetooth in my car is connected?

I have a Nexus 5 running Lollipop, OK Google is enabled on all screens and works well. Except, that is, when the phone is connected to my car (2013 VW Jetta). In this state, voice search has the unfilled microphone. Even if I touch the mic icon it initializes and cancels the search immediately.

The car has voice commands, but not via the phone. There is no way (that I know of) to use the car to initiate a search on the phone. Only music commands and calling a contact.

I have tried enabling and disabling the Bluetooth headset option in the voice settings.